A fuseboard I was looking at has a spare way, but it is not rated up to 40A, only 30A,so I was going to replace the board. I contacted a website and they told me that instead of replacing it I could add a shower unit (containing a 30mA RCD and a 40A MCB) if there was a separate main switch. The cables supplied with the shower unit are 10mm tails. What size should the earth be?